Turbine engines generally produce a high level of noise across a broad spectrum, and the turbofan engine is the dominant contributor to overall aircraft noise. As such, reduction of turbofan noise is important for aircraft compliance with current and future noise regulations. Fan noise has traditionally been reduced by a combination of passive liner treatments and nacelle modifications. Conventional, passive liners generally consist of a honeycomb core bonded between a porous facesheet and an impervious backplate. Such a configuration produces an array of independent, one-dimensional, tuned waveguides that behave as local-reacting absorbers. The acoustic absorption spectra of such structures are characterized by a single peak at the system resonance frequency and its odd harmonics with significantly reduced absorption at other frequencies.
In an attempt to increase bandwidth, a Helmholtz resonator could be used to reduce noise in lower frequencies, in which the volume of the chamber is sufficiently large to allow absorption of the low frequency. However, this typically means that all of the available volume is used to reduce low frequency noise, and there is insufficient additional volume for liner components targeting the higher frequencies. Another approach to broadband noise reduction is to provide an acoustic liner having cells of variable height, where the cells are arranged from shortest height to tallest height, gradually increasing with each successive cell. Acoustic liner 100 of FIG. 1 shows such a design, where cells 110 vary in height from 1.5 inches to 12 inches. However, such configurations have unacceptable thickness for many turbine engine designs. For instance, if the cells of the acoustic liner are all oriented at a steep angle with respect to the nacelle/exhaust nozzle, as is the case in FIG. 1, then the liner may become unacceptably thick. In particular, this is a significant concern in the core region of the engine, where the volume available for an acoustic liner is limited and the trend tends to be for reduction of core cross-sectional area. Accordingly, an acoustic liner that can absorb a wide range of frequencies without having undue thickness may be beneficial.